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February 19, 2005

How the Fruitfly Got Its Spots

by Sam Savage

The evolutionary mystery of how a leopard got its spots illustrates the challenges of tracing changes in form and pattern. A new model system in fruitflies shows how colorful decoration may enhance reproductive chances for success. It turns out that mutations may stick around if they don't get the organism killed first.

Astrobiology Magazine -- Like the gaudy peacock or majestic buck, the bachelor fruit fly is in a race against time to mate and pass along its genes. And just as flashy plumage or imposing antlers work to an animal's reproductive advantage, so, too, do the colored spots that decorate the wings of a particular male fruit fly.

To the ladies, the spots - waved frenetically by suitors in the fruit fly courtship ritual - connote sex appeal.

To a team of Wisconsin scientists, however, the origin of these decorative spots has proven to be a critical portal to unraveling a long-standing genetic mystery: What is it, exactly, that governs the development and evolution of form? Is it the genes themselves or the devices within DNA that control where genes are used in the making of the animal's body?

The answer, according to the team from the Howard Hughes Medical Research Institute (HHMI) at the University of Wisconsin-Madison, and published this week (Feb. 2) in the journal Nature, is that the heavy lifting of evolution is accomplished through changes in the genetic switches that direct how genes work.

"This is smoking gun evidence of how animal patterns evolve," says Sean B. Carroll, a UW-Madison professor of genetics and the senior author of the Nature paper.

While discovered in flies, it is almost certain that the same mechanisms are at play in all other animals, including humans, and help determine everything from the snout of an aardvark to the stripes of the zebra.

The discovery is important because it provides critical evidence of how animals evolve new features to improve their chances of reproductive success and survival. It is, says Carroll, convincing proof that evolution occurs as accidental mutations create features - a spot here or a stripe there - that confer advantages in attracting mates, hiding from or confusing predators, or gaining access to food. The mutations are preserved, according to the Nature report, as changes in a few of the millions of nucleotides - the chemical building blocks of DNA.

The fruit fly, says Carroll, proved to be ideal to study the fine print of evolution because one species, Drosophila melanogaster, is among the most-studied animals in biology and is a workhorse of modern genetics. But there are thousands of species of fruit fly, and unlike the very plain melanogaster, the wings of males of many species sport decorations that are as diverse and as beautiful as the wings of butterflies.

The data from the Wisconsin study, Carroll and his colleagues say, confirm the long-held idea that "evolution is a combination of chance and ecological necessity, which selects those things that are going to be kept. It means that (an animal's features) are just accidents - accidents that are preserved" because they confer some kind of advantage.

While the decorations on a fruit fly's wing begin as accidents of development, the patterns we see are far from willy-nilly murals of nature. The spots tend to occur along physical landmarks of the wing, at the junctures of veins, which provide contours and boundaries like those of a leaded-glass window.

"The patterns on a wing are not just random graffiti. The spots emerge at specific places," Carroll says.

"The structure of the fruit fly wing has been around for a long time, and the (different species) paint by parameters that are already there," says Nicolas Gompel, who, along with Benjamin Prud'homme, is a lead author of the new Nature report. "What we see in many different species is the repeated use of a pattern that is already built into the wing," Gompel says.

The new work by the Wisconsin HHMI team adds to the accumulating understanding of how evolution works at the most fundamental level, says Carroll. "The depth of our understanding of evolution is only growing," he says.